1. Field of the Invention
The present invention relates to the rectification of solvents of the chlorinated hydrocarbon series to enable them to be used for the treatment of noble metals, such as copper or silver, without causing discoloration of the surface of these metals.
2. Discussion of the Prior Art
Chlorinated hydrocarbons such as trichlorethylene and perchlorethylene are used for the cleaning, degreasing or drying of metal objects. This treatment can be performed with the said solvents either in the liquid phase or in the vaporous phase. For the practical use of the chlorinated hydrocarbons they must be stabilized to protect them against degradation or prevent them from attaching the articles being treated. Such degradation is caused, for example, by the oxidizing action of atmospheric oxygen on the double bond of perchlorethylene or trichlorethylene, as the case may be. The hydrogen chloride that is thus released attacks the metal surfaces. To forestall this phenomenon, the solvents must, after rectification by distillation, be carefully neutralized, dried, and then stabilized. Phenols, amines, dioxane, nitro compounds, esters, nitriles and epoxides, for example, have been described as stabilizers.
Depending on the method by which they have been produced, in some cases a special treatment of the chlorinated hydrocarbons is necessary prior to their stabilization. For example, in German "Offenlegungsschrift" No. 2,111,735 a description is given of the preliminary treatment of chlorinated hydrocarbons with diamines for the purpose of removing oxidizing compounds which otherwise would call for an excessively great consumption of stabilizer. This treatment with diamines is especially necessary when the chlorinated hydrocarbons have been produced by an oxychlorination process and contain oxygenous compounds of the type Cl--R--O--OR--Cl or ##STR1##
With or without this treatment, it is possible sufficiently to stabilize chlorinated hydrocarbons, especially unsaturated chlorinated hydrocarbons. The use of the substances named above generally suffices to protect these solvents and thus also the metal surfaces being treated.
If, however, objects made of valuable metals, such as silver or copper, in which the external appearance of the metal surface after hot or cold treatment with the solvents is important, are treated with the above-named already stabilized solvents and cleaning agents, the mere addition of the stabilizers described in the literature is often insufficient. Thus it is often observed that copper surfaces are darkened and silver surfaces acquire a yellowish discoloration when they are subjected to treatment with hot chlorinated hydrocarbons, for the purpose of drying them, for example. Particularly in the case of metal surfaces of decorative value, this effect is most undesirable.
The surface discoloration effect occurs especially when a chlorinated hydrocarbon is involved which has been produced from a variety of prechlorinated products of different composition. These chlorinated hydrocarbon mixtures originate in chemical plants in which vinyl chloride is produced in a series of steps from ethylene, chlorine and hydrogen chloride by chlorination, oxychlorination and dehydrochlorination. In this process a plurality of more or less chlorinated and partially also oxychlorinated hydrocarbons are formed as by-products which must be separated, by distillation for example, and destroyed. However, they can also be used alone or together with 1,2-dichlorethane in a catalytic high-temperature chlorination for the production of perchlorethylene and/or trichlorethylene. As a result of the multiplicity of the components which in this case react with chlorine, compounds are also formed in small amounts which, when the solvents are used on copper or silver, produce the tarnishing action described above. This undesirable effect is not prevented by the sole addition of the conventional stabilizers.
One method of preventing the "tarnishing" of copper objects upon treatment with chlorinated hydrocarbons is described in German "Offenlegungsschrift" No. 2,036,939, in which the presence of sulfur compounds in the chlorinated hydrocarbons is given as the cause. The elimination of these undesirable sulfur compounds is accomplished in accordance with the method described in that specification by the addition of phenol, cresol or xylenol. The sulfur compounds in chlorinated hydrocarbons are not, however, the only cause of the above-named tarnishing of the noble metals. Even if sulfur-free chlorinated hydrocarbons such as those obtained by the vinyl chloride production processes described above are treated with phenols, the discoloration of the metal surfaces is not prevented.